Fix dirname detection on Windows systems

[zedxxx]

On Windows systems function dirname fails and always return .
This fix replace all Windows backslashes \ to Unix forward slashes / in script path, so dirname return proper dir.

[frej]

Sorry, I will not commit this. \ is a perfectly valid (if uncommon) filename component on unix. To me this is a bug in dirname, not fast-export.

[zedxxx]

No bugs in dirname, because it's work only with /:

Usage: dirname NAME
  or:  dirname OPTION
Print NAME with its trailing /component removed; if NAME contains no /'s,
output '.' (meaning the current directory).

  --help      display this help and exit
  --version   output version information and exit

Report bugs to <bug-sh-utils@gnu.org>.

Is any idea about how can I fix ROOT detection in *.sh script on Windows?